Method for synthesizing trioxane

ABSTRACT

A method for synthesizing trioxane by heating and reacting an aqueous formaldehyde solution in the presence of a heteropolyacid. As the heteropolyacid, there is preferably used one that contains, as a coordinating element or coordinating elements, W, Mo, V or Nb alone or two or more of them.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for synthesizing trioxane,characterized by heating and reacting formaldehyde in the presence of aheteropolyacid.

2. Description of the Prior Art

This invention relates to a method for synthesizing trioxane fromformaldehyde in the presence of a heteropolyacid.

Trioxane is a cyclic trimer of formaldehyde and is used mainly as astarting material for producing polyoxymethylene. In order to obtainpolyoxymethylene having a satisfactory molecular weight, very puretrioxane is required and it is necessary that its contents inparticular, of the so-called by-products such as water, formic acid andmethanol which become chain transfer agents, methylal, methyl formate,and polyoxymethylenedimethoxide with a low molecular weight, and thelike should be low.

Many researches have been done on a method for synthesizing trioxanesatissfying these requirement, though a usual production method consistsof obtaining trioxane by heating formaldehyde in the presence of an acidcatalyst, for example, sulfuric acid, phosphoric acid, boric acid,benzenesulfonic acid, toluenesulfonic acid, an acidic ion-exchangeresin, or the like, or in the presence of a solid acid catalyst such asaluminum sulfate, silica or the like. Among the above-mentionedcatalysts, sulfuric acid is most generally used becuse the reaction rateis fast when it is used, and it is easily available. However thesulfuric acid method have involved several practical problems to besolved. For example, when the formaldehyde concentration exceeds 60% byweight, paraformaldehyde is formed, and when the sulfuric acidconcentration is 8% by weight or higher, by-products such as formicacid, methyl formate and the like are formed so that the yield oftrioxane is lowered. Further, when the sulfuric acid concentration is10% by weight or higher, many by-products are formed, therefore it hasbeen proposed to effect the reaction by adding a dispersing agent suchas di-2-ethylhexyl phthalate, however the addition of the thirdcomponent makes post-treatement and the like troublesome and hence isnot thought to be a suitable method. As mentioned above, conventionalmethods have had many disadvantages for application on an industrialscale, for example, (1) many by-products are formed, (2 ) scales ofparaformaldehyde tend to be formed in th reactor or on the wall of thedistilling column, (3) the reactor and the wall of the distilling columnare corroded.

SUMMARY OF THE INVENTION

The object of this invention is to find a reaction method free fromvarious defects which conventional method have in the production oftrioxane from formaldehyde, for example, (1) many by-products areformed, (2) scales of paraformaldehyde tend to be formed in the reactoror on the wall of the distilling column, (3) the reactor and the wall ofthe distilling column are corroded.

In order to accomplish the above-mentioned object, the present inventorshave made various studies on the reaction method to find that whenformaldehyde is heated and reacted in the presence of a heteropolyacid,the object can be accomplished, that is to say, (1) the trioxane contentin the distillate obtained by distilling the reaction solution after thereaction is high, and its contents of by-products such as formic acid,methyl formate, methylal, methanol and the like are low, (2) when a highconcentration of a heteropolyacid is present, the solubility offormaldehyde is increased so that it becomes possible to react anaqeuous formaldehyde solution having a high concentration, therefore aproduct high in content of trioxane, can be obtained, (3) noparaformaldehyde is deposited in the reactor or on the wall of thedistilling column, (4) the reactor, the wall of the distilling columnand the like are not corroded: therefore it is very advantageous forpractical purposes.

DETAILED DESCRIPTION OF THE INVENTION

This invention is most characteristic in that a heteropolyacid is usedas a reaction catalyst. This heteropolyacid is as shown in "Catalyst"vol. 18, No. 6, p. 169-177(1976), one having a mononuclear orpolynuclear complex ion which has a diverse element (central element) inthe center and is formed by the condensation of acid radicals withsharing an oxygen atom. The heteropolyacid is generally represented bythe following chemical formula:

    H.sub.n [M.sub.x M'.sub.y O.sub.z ].mH.sub.2 O

wherein M is a central element, M' is a coordinating element and isgenerally one or more elements selected from the group consisting of W,Mo, V and Nb, x is a value of 0.1 to 10, y is a value of 6 to 30, zrepresents the number of oxygen atoms in the heteropolyacid and is avalue in the range from about 10 to 80, n represents the number ofacidic hydrogen atoms in the heteropolyacid and is a value larger than1, and m represents the number of moles of water of crystallization andhas a value in the range from 0 to about 40. The so-called mixedheteropolyacids containing two or more kinds of coordinating elementsare also included in the heteropolyacids of this invention.

The central element M in the above-mentioned chemical formula generallyconsists of one or more elements selected from the group consisting ofP, B, Si, Ge, Sn, As, Sb, U, Mn, Re, Cu, Ni, Co, Fe, Ce, Th and Cr.Heteropolyacids which are particularly preferably used are those inwhich the central element M is P, Si, B, Ge, Cu or Sn, and among themthose in which M is Si or P are particularly preferred.

The coordinating element M' in the above-mentioned chemical formulaconsists of one or more elements selected from the group consisting ofW, Mo, V and Nb, though heteropolyacids which are particularlypreferably used are those in which the coordinating element M' is W, Mo,V or a mixture thereof. Any of heteropolyacids within theabove-mentioned range can accomplish the object of this invention,though particularly preferable ones among them are tungstosilicic acid,molybdosilicic acid, tungstophosphric acid, molybdophosphoric acid andmixtures thereof.

When these heteropolyacids are used, the trioxane content in thedistillate obtained by distilling the reaction solution after thereaction is high, and its contents of by-products such as formic acid,methyl formate, methylal, methanol and the like are low. Thedistillation referred to here is distillation at the boiling point ofthe reaction solution.

In this invention, hydrous formaldehyde having a formaldehydeconcentration of 95% by weight or lower may be used as a startingmterial. Needless to say, there may also be used, as starting meterial,compounds from which formaldehyde is formed by heating, such asparaformaldehyde, α-polyoxymethylene and the like. In particular, inorder to practice this invention effectively, it is preferable to usehydrous formaldehyde having a formaldehyde concentration of 30 to 80% byweight.

In the reaction of this invention, formaldehyde is heated and reacted inthe presence of a heteropolyacid, and the reaction conditions are asfollows. The proportion of the heteropolyacid to the starting materialin the reactor is usually 5 to 600 parts by weight, preferably 30 to 400parts by weight, particularly preferably 100 parts by weight or more ofa heteropolyacid to 100 parts by weight of a 30-80 wt% aqueous solutionof formaldehyde. The reaction temperature at the time of reaction underheating is preferably 60° to 200° C., though an effective method is toeffect the reaction by heating at the boiling point of the reactionsolution, i.e., at about 100° C. However, the boiling point varies whenpressure is applied or the pressure is reduced, and it varies dependingalso upon the proportion of formaldehyde, water and a heteropolyacid toone another in the reaction solution.

Next, the advantages of this invention are described below.

In the first place, the selectivity and the degree of conversion arehigh in the reaction according to this invention (the selectivityreferred to here means the proportion of trioxane to all the productsdistilled out, and hence is expressed by the equation: ##EQU1## and thedegree of conversion means the proportion of the reacted formaldhyde tothe starting formaldehyde). For example, when, as shown in Example 1, 10parts by weight of tungstosilicic acid is added to a 60% by weightaqeuous formaldehyde solution and the resulting mixture is heated andreacted at atmospheric pressure, the sum of the amounts of by-products,methylal, methyl formate, formic acid and methanol may be limited to 1%or less and side reactions are greatly suppressed as compared with thecase of using a sulfuric acid catalyst which is usually used. This meansthat no impurities accumulate in the reactants, and hence is veryadvantageous for purifying the trioxine. The trioxane content in thedistillate taken out from the reactor is high, which means that theamount of the unreacted material is small, and hence is not onlyadvantageous for purifying trioxane but also economical of energy. Asmentioned above, the method of this invention is advantageous forindustrial application.

In the second place, even when the concentration of startingformaldehyde to be used is high, no precipitate of paraformaldehydeadheres to the inside of the reactor. In the case of using a sulfuricacid catalyst which is usually used, when the formaldehyde concentrationat the time of the reaction is increased, for example, to 60% by weightor higher, precipitates and scales of paraformaldehyde are formed in thereactor, and when the sulfuric acid concentration is increased in orderto increase the formaldehyde concentration to 60% by weight or higher,the production of by-products is greatly increased and this becomes aproblem in effecting the reaction on an industrial scale. On the otherhand, according to the method of this invention, the formaldehydeconcentration in the reaction solution can be increased, so that thetrioxane content in the vapor distilled out can be increased. This isbecause the solubility of formaldehyde increases in proportion as theconcentration of the heteropolyacid increases, and because when a highconcentration of a heteropolyacid is allowed to coexist withformaldehyde, it becomes possible to synthesize trioxane from an aqueousformaldehyde solution having a higher concentraiton. Furthermore, forexample, when a heteropolyacid is present in an amount of 100 parts byweight or more per 100 parts by weight of an aqeuous formaldehydesolution having a concentration of 30 to 80% by weight, it also becomespossible to synthesize trioxane from an aqueous formaldehyde solutionhaving a concentration of 60% by weight or higher, for example, 80% byweight without the formation of precipitates of paraformaldehyde. It ispresumed that the heteropolyacid thus acts as a catalyst for thesynthesis of trioxane from formaldehyde, however it is difficult to saythat the mechanism of its action has become clear. According to themethod mentioned above, the water content in the product distilled outfrom the reactor is decreased so that trioxane having a very highconcentration can be obtained, and no paraformaldehyde precipitates inthe reactor, therefore an industrially advantageous process can berealized.

In the third place, the heteropolyacid used in this invention does notcorrode the material for the apparatus which is industrially used. It isa fact universally known by those skilled in the art that wellknownstrong acid catalysts which are generally used are remarkably corrosiveunder the reaction conditions, therefore other measures are necessaryfor industrial application, and this is an important problem. However,heteropolyacids are noncorrosive at least at a concentration within therange mentioned in this invention, and cause no troubles in practicalapplication.

In practicing this invention industrially, it is proper to use arectifying column by connecting it onto the reactor containing acatalyst. When a starting aqeuous formaldehyde solution is continuouslyfed to the reactor at a constant flow rate, a vapor or solutioncontaining trioxane can be taken out from the top of the rectifyingcolumn at a flow rate corresponding to the feeding rate. In the reactor,the aqueous formaldehyde solution is boiling in the presence of thecatalyst, however the catalyst is hardly effused from the reactor,therefore when the amount of the reaction solution in the reactor iskept constant, the operation can be performed keeping the averagereaction time constant. The proportion of the catalyst to the startingmaterial mentioned above can be shown as a proportion in the reactoreven in the case of such a flow reaction.

This invention is further explained below in more detail referring toExamples.

EXAMPLE 1

To a reactor where charged 100 parts by weight of a 60% by weightaqueous solution (containing 0.5% by weight of methanol) of formaldehydeprepared from commercially available paraformaldehyde and 10 parts byweight of tungstosilicic acid, and they were heated and distilled. Arectifying column was attached to the reactor, after which a 60% byweight aqueous formaldehyde solution was continuously fed to the reactorso that the amount of the reaction solution in the reactor might alwaysbe constant, and the reaction solution was heated so that thetemperature of the distilled might be 93° to 97° C. The distillate wassampled and examined for its composition. The results 3 hours after theinitiation of the reaction are shown in Table 1. For comparison, anexperiment was carried out in the same manner as above except that inplace of the tungstosilicic acid, 10 parts by weight of sulfuric acidwas allowed to coexist with the formaldehyde. As is obvious from Table1, the use of tungstosilicic acid resulted in not only a higher trioxanecontent in the product, i.e., a higher degree of conversion in thereaction but also less side reactions, as compared with the use ofsulfuric acid.

                  TABLE 1                                                         ______________________________________                                                      Tungstosilicic                                                                             Sulfuric acid*                                     Catalyst      acid (10 parts by                                                                          (10 parts by                                       Component     weight)      weight)                                            ______________________________________                                        Trioxane      42.0         31.4                                               Water         39.5         40.0                                               Formaldehyde  18.0         25.0                                               Methyl formate                                                                              0.01         1.2                                                Methylal      0.01         0.8                                                Methanol      0.5          1.2                                                Formic acid   --           0.4                                                ______________________________________                                         Percentage of each component is by weight.                                    *is a comparative example.                                               

EXAMPLE 2

Each of various heteropolyacids was allowed to coexist with 100 parts byweight of a 60% by weight aqueous formaldehyde solution, and thecomposition of the product was examined by using the same method as inExample 1. The reaction was effected while controlling the temperatureof the vapor distilled out so as to be 93° TO 97° C. The results 3 hoursafter the initiation of the reaction are shown in Table 2. Further, theformation of scales in the reaction system was also examined. Theselectivity referred to in Table 2 means, as described above, theproportion of trioxane to all the products distilled out and hence isexpressed by the equation: ##EQU2##

                                      TABLE 2                                     __________________________________________________________________________                    Composition of the reaction                                                   solution        Results                                                                       Trioxane                                                      Catalyst acid                                                                        Aqueous form-                                                                          content in                                                    (parts by                                                                            aldehyde solution                                                                      the distillate                                                                        Selectivity                                                                         Formation                       Catalyst acid   weight)                                                                              (parts by weight)                                                                      (% by weight)                                                                         (%)   of scales                       __________________________________________________________________________    Tungstosilicic acid                                                                           30     100      44      99.86 none                            (H.sub.4 SiW.sub.12 O.sub.40.XH.sub.2 O)                                      Tungstosilicic acid                                                                           100    100      46      99.78 none                            (H.sub.4 SiW.sub.12 O.sub.40.XH.sub.2 O)                                      Tungstophosphoric acid                                                                        30     100      43      99.80 none                            (H.sub.3 PW.sub.12 O.sub.40.XH.sub.2 O)                                       Tungstophosphoric acid                                                                        100    100      45      99.81 none                            (H.sub.3 PW.sub.12 O.sub.40.XH.sub.2 O)                                       Molybdosilicic acid                                                                           30     100      42      99.85 none                            (H.sub.4 SiMo.sub.12 O.sub.40.XH.sub.2 O)                                     Molybdosilicic acid                                                                           100    100      46      99.82 none                            (H.sub.4 SiMo.sub.12 O.sub.40.XH.sub.2 O                                      Molybdophosphoric acid                                                                        30     100      41      99.88 none                            (H.sub.3 PMo.sub.12 O.sub.40.XH.sub.2 O)                                      Molybdophosphoric acid                                                                        100    100      45      99.80 none                            (H.sub.3 PMo.sub.12 O.sub.40.XH.sub.2 O)                                      Vanadosilicic acid                                                                            30     100      37      99.70 none                            Vanadophosphoric acid                                                                         30     100      36      99.67 none                            Tungstogermanic acid                                                                          30     100      36      99.55 none                            Tungstoboric acid                                                                             30     100      34      99.52 none                            Molybdocupric acid                                                                            30     100      35      99.51 none                            Molybdostannic acid                                                                           30     100      36      99.57 none                            Vanadomolybdophosphoric acid                                                                  30     100      37      99.58 none                            (H.sub.4 PVMo.sub.11 O.sub.40.XH.sub.2 O)                                     Sulfuric acid   30     100      34      95.30 formed                          (Comparative Example)                                                         Sulfuric acid   50     100      36      93.15 formed                          (Comparative Example)                                                         __________________________________________________________________________

EXAMPLE 3

Various parts by weight of tungstosilicic acid or tungstophosphoric acidwas added to 100 parts by weight of a 60% by weight aqueous formaldehydesolution, and the composition of the product was examined by the samemethod as in Example 2. The temperature of the distillate was controlledso as to be 93° to 97° C., and the reaction results 3 hours after theinitiation of the reaction are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                       Results                                                                             Gaseous                                                                       trioxane                                                            Concent-  concent-                                                            ration    ration    Selec- Forma-                                             (parts by (% by     tivity tion of                                 Heteropolyacid                                                                           weight)   weight)   (%)    scale                                   ______________________________________                                        Tungstosilicic                                                                           3         30        99.91                                          acid       5         37        99.85                                          (H.sub.3 SiW.sub.12 O.sub.40.                                                            10        42        99.88                                          XH.sub.2 O)                                                                              30        44        99.86                                                     100       46        99.78  none                                               200       47        99.75                                                     300       48        99.70                                                     400       48        99.75                                                     500       47        99.70                                                     600       45        99.65                                          Tungstophos-                                                                             3         29        99.90                                          phoric acid                                                                              5         36        99.86                                          (H.sub.3 PW.sub.12 O.sub.40.                                                             10        41        99.84                                          XH.sub.2 O)                                                                              30        43        99.80                                                     100       45        99.81  none                                               200       47        99.78                                                     300       47        99.67                                                     400       47        99.70                                                     500       46        99.65                                                     600       45        99.67                                          ______________________________________                                    

EXAMPLE 4

Aqueous formaldehyde solutions were prepared so that the concentrationof formaldehyde might be 25, 30, 40, 55, 60, 64, 70, 73, 75 and 80% byweight, respectively, and 400 parts by weight of tungstosilicic acid wasadded to 100 parts by weight of each of said aqueous solutions, afterwhich the resulting mixture was heated and reacted in a reactor, and thecomposition of the product was examined by the same method as inExample 1. The reaction was effected while controlling the temperatureof the vapor distilled out so as to be 93° to 97° C., and the reactionresults 3 hours after the initiation of the reaction are shown in Table4. In cases of all the formaldehyde concentrations tested, formaldehydewas uniformly dissolved.

                  TABLE 4                                                         ______________________________________                                        Concentration of                                                                            Results                                                         starting form-                                                                              Concentration of                                                aldehyde      product trioxane                                                                           Selectivity                                        (% by weight) (% by weight)                                                                              (%)                                                ______________________________________                                        25            10           99.91                                              30            18           99.95                                              40            26           99.90                                              55            41           99.85                                              60            48           99.87                                              64            53           99.84                                              70            61           99.67                                              73            64           99.48                                              75            67           99.42                                              80            68           99.40                                              ______________________________________                                    

EXAMPLE 5

Each of various kinds of aqueous acid solutions was placed in a vesselmade of SUS 304 and heated in an oil bath at 140° C., and the conditionof the surface of the vessel which was in contact with the acid solutionwas observed with the lapse of time. In the case of an aqueous sulfuricacid solution, the SUS surface lost its luster with the lapse of time,and was corroded. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                        Corrosion of SUS surface                                      Acid              1 hour   5 hours  3 days                                    ______________________________________                                        Sulfuric acid  5%     +        +      +++                                     Sulfuric acid 30%     ++       +++    +++                                     Tungstosilicic acid                                                                         10%     -        -      -                                       Tungstosilicic acid                                                                         30%     -        -      -                                       Tungstosilicic acid                                                                         90%     -        -      -                                       Tungstophosphoric acid                                                                      90%     -        -      -                                       Molybdosilicic acid                                                                         90%     -        -      -                                       Molybdophosphoric acid                                                                      90%     -        -      -                                       Tungstogermanic acid                                                                        90%     -        -      -                                       Vanadosilicic acid                                                                          90%     -        -      -                                       ______________________________________                                         +express barely detectable corrosion                                          ++express clearly detectable corrosion                                        +++express serious corrosion                                                  -express a negative result, i.e., no corrosion                           

EXAMPLE 6

Each of various kinds of aqueous acid solutions was placed in a vesselmade of SUS 304 and heated in an oil bath at 140° C., and the change ofcolor of the aqueous acid solution was observed with the lapse of time.In the case of an queous sulfuric acid solution, the solution wascolored (blue) by the release of metal ions which was caused by thecorrosion of SUS. The results are shown in Table 6, wherein + expressescoloration, the number of + expresses the degree of coloration, and -expresses a negative result, i.e., no coloration.

                  TABLE 6                                                         ______________________________________                                                        Coloration of an aqueous                                                      acid solution                                                 Acid              1 hour   5 hours  3 days                                    ______________________________________                                        Sulfuric acid  5%     +        +      +++                                     Sulfuric acid 30%     +++      +++    +++                                     Tungstosilicic acid                                                                         90%     -        -      -                                       Tungstophosphoric acid                                                                      90%     -        -      -                                       ______________________________________                                    

What is claimed is:
 1. A method of synthesizing trioxane which comprisesheating formaldehyde at a temperature from 60° to 200° C. in thepresence of an aqueous solution of a heteropolyacid selected from thegroup consisting of tungstosilicic acid, tungstophosphoric acid andmixtures of these acids.
 2. A method for synthesizing trioxane accordingto claim 1, wherein the formaldehyde is hydrous formaldehyde having aconcentration of 30 to 80% by weight.
 3. A method for synthesizingtrioxane according to claim 2, wherein the heteropolyacid is used in anamount of 5 to 600 parts by weight per 100 parts by weight of theaqueous formaldehyde solution.
 4. A method for synthesizing trioxaneaccording to claim 2, wherein the heteropolyacid is used in an amount of100 to 600 parts by weight per 100 parts by weight of the aqueousformaldehyde solution.